I. Field of the Invention
This invention relates generally to the treatment of molten metals with gases prior to casting or other processes involving metal cooling and solidification, to remove dissolved gases (particularly hydrogen), non-metallic solid inclusions and unwanted metallic impurities prior to cooling and solidification of the metal. More particularly, the invention relates to gas injectors, and apparatus employing such injectors, used for the treatment of molten metals in this way.
II. Description of the Prior Art
Many molten metals used for casting and similar processes must be subjected to a preliminary treatment to remove unwanted components that may adversely affect the physical or chemical properties of the resulting cast product. For example, molten aluminum and aluminum alloys derived from alumina reduction cells or metal holding furnaces usually contain dissolved hydrogen, solid non-metallic inclusions (e.g. TiB.sub.2, aluminum/magnesium oxides, aluminum carbides, etc.) and various reactive elements, e.g. alkali and alkaline earth metals. The dissolved hydrogen comes out of solution as the metal cools and forms unwanted porosity in the product. Non-metallic solid inclusions reduce metal cleanliness, and the reactive elements and inclusions create unwanted metal characteristics.
These undesirable components are normally removed from molten metals by introducing a gas below the metal surface by means of gas injectors. As the resulting gas bubbles rise through the mass of molten metal, they adsorb gases dissolved in the metal and remove them from the melt. In addition, non-metallic solid particles are swept to the surface by a flotation effect created by the bubbles and can be skimmed off. If the gas used for this purpose is reactive with contained metallic impurities, the elements may be converted to compounds by chemical reaction and removed from the melt in the same way as the contained solids or by liquid-liquid separation.
This process is often referred to as "metal degassing," although it will be appreciated from the above description that it may be used for more than just degassing of the metal. The process is typically carried out in one of two ways: in the furnace, normally using one or more static gas injection tubes; or in-line, by passing the metal through a box situated in the trough normally provided between a holding furnace and the casting machine so that more effective gas injectors can be used. In the first case, the process is inefficient and time consuming because large gas bubbles are generated, leading to poor gas/metal contact, poor metal stirring and high surface turbulence and splashing. Dross formation and metal loss result from the resulting surface turbulence, and poor metal stirring results in some untreated metal. The second method (as used in various currently available units) is more effective at introducing and using the gas. This is in part because the in-line method operates as a continuous process rather than a batch process.
For in-line treatments to work efficiently, the gas bubbles must be in contact with the melt for a suitable period of time and this is achieved by providing a suitable depth of molten metal above the point of injection of the gas, and by providing a means of breaking up the gas into smaller bubbles and dispersing the smaller bubbles more effectively through the volume of the metal, for example by means of rotating dispersers or other mechanical or non-mechanical devices. Metal residence times in the containers in which such degassing operations are performed are often in excess of 200 seconds, and frequently in excess of 300 seconds.
Effectiveness of degassing is frequently defined in terms of the hydrogen degassing reaction for aluminum alloys and an adequate reaction is generally considered to be one achieving at least 50% hydrogen removal (typically 50 to 60%). This results in the need for deep treatment boxes of large volume (often holding three or more tons of metal) which are unfortunately not self-draining when the metal treatment process is terminated. This gives rise to operational problems and the generation of waste because metal remains in the treatment boxes when the casting process is stopped for any reason and solidifies in the boxes if not removed or kept molten by heaters. Moreover, if the metals or alloys being treated are changed from time to time, the reservoir of a former metal or alloy in a box (unless it can be tipped and emptied) undesirably affects the composition of the next metal or alloy passed through the box until the reservoir of the former metal is depleted.
The entry and exit sections of such degasser boxes generally have cross-sectional areas (measured in a vertical plane orientated transversely to the direction of metal flow) substantially less than the corresponding cross-sectional area of the degasser box itself in order to match the cross-sectional area of the metallurgical troughs used to feed metal to and remove metal from the degasser box. Thus, when the degassing operation is ended and the metal flow is interrupted (resulting in draining of the metallurgical troughs), almost all the metal in the degasser box is retained and must be maintained in the molten state by operating heaters, ladled or pumped out, or poured out by mechanically tilting the entire degasser box.
Various conventional treatment boxes are in use, but these require bulky and expensive equipment to overcome these problems, e.g. by making the box tiltable to remove the metal and/or by providing heaters to keep the metal molten. As a consequence, the conventional equipment is expensive and occupies considerable space in the metal casting facility. Processes and equipment of this type are described, for example, in U.S. Pat. Nos. 3,839,019 and 3,849,119 to Bruno et al.; 3,743,263 and 3,870,511 to Szekeley; 4,426,068 to Gimond et al.; and 4,443,004 to Hicter et al. Modern degassers of this type generally use less than one liter of gas per kilogram (Kg) of metal treated. In spite of extensive development of dispersers to achieve greater mixing efficiency, such equipment remains large, with metal contents of at least 0.4 m.sup.3 and frequently 1.5 m.sup.3 or more being required. One or more dispersers such as the rotary dispersers previously mentioned may be used, but for effective degassing, at least 0.4 m.sup.3 of metal must surround each disperser during operation.
U.S. Pat. No. 5,527,381 to Waite et al. describes a degasser in which the box-like structure of the earlier devices is replaced by a section of trough having approximately the same cross-sectional area as the metallurgical troughs feeding and removing metal from the degasser. This creates a degasser of smaller volume and one which retains little if any metal when the source of metal is interrupted after the degassing operation is completed (i.e. it is substantially self-draining along with the trough). The degasser uses several relatively small rotary gas injectors along the length of a trough section to achieve the equivalent of a continuous "plug" flow reactor, giving a high degassing efficiency.
To achieve effective degassing, all degassing apparatus must deliver a certain minimum volume of gas per kilogram of metal, and in a trough-like vessel where the residence time of the metal in the region in which the gas is supplied is substantially less than in the deep box degassers, the amount of gas which each rotary injector must deliver is high and the ability to deliver a suitable amount of gas determines the effectiveness of an injector design.
The gas injectors disclosed in U.S. Pat. No. 5,527,381 to Waite et al. are capable of delivering a suitable volume of gas to a molten metal and are consequently capable of effective use in trough-like degassers, as described in the patent. However, it has been noticed that gas tends to be released from such rotors in an irregular manner, causing splashing at the surface of the molten metal and inefficiency of dissolved gas removal.
There is a need, therefore, for a compact rotary gas injector capable of delivering large volumes of gas in the form of fine bubbles into molten metal without substantial irregularities of-gas flow, suitable for use in trough-like degassers or in any application in which such high gas delivery in the form of fine bubbles is required.